# Are my calculations correct for a swimming pool?

Discussion in 'Swimming Pools and Hot Tubs' started by ecosystem3, Jun 29, 2009.

1. ### ecosystem3New Member

I’m looking to put in a geothermal heat pump to heat my parents swimming pool. It’s 40’x20’ and 27,000 gallons. The target water temperature is 85 degrees and I’m basing the average air temperature at 60 degrees assuming they will be using the pool earlier in the year (May-September). The house is in the tri-state New York area. My calculations result in needing 100,000 BTU’s to maintain an 85 degree pool.

hsurface = (5 Btu/hr ft2 oF) ((85oF) - (60oF)) (40 ft) (20 ft)

Am I sizing the system correctly? Thanks for any help provided.
Greg

4. ### engineerWell-Known MemberIndustry ProfessionalForum Leader

The relevant variables are:

1) Pool surface area

2) desired pool temp degrees above average ambient

3) Wind speed

5. ### GuestGuest

Plus humidity.

6. ### urthbuoyWell-Known MemberIndustry ProfessionalForum Leader

From ASHRAE, 1999a

Your heat losses are:
1. Conduction through pool wall
2. Convection on the surface
3. Radiation from the surface
4. Evaporation from the surface

Conduction is relatively insignificant unless it is an aboveground pool. Most of the issue is evaporation losses that can be dealt with by pool covers or indoor pools.

If you ignore a pool heat up time calculation you are then left with.

Q (Kj/h) = U(h m2 C)* A (m2) * (Tpool(C)-Tambient(C))
- forgot to add U (surface heat transfer coefficient) = 214.4 KJ/(h m2 C)

This assumes average wind velocity of 5-8km/hr.

For more details on this - Google the source: Design Considerations for Pools and Spas (Natatoriums), John W. Lund, Geo-Heat Center, GHC Bulletin, September 2000.

7. ### TedKiddNew Member

Wow, that surprises me! Lot of 85f surface area directly connected to 50f ground (unless the pool has insulation)! Can you help me understand this/tell me more?

This makes a lot of sense, particularly for low humidity areas.

8. ### SemiRetired EngineerNew Member

If this is a rectangular pool (or not too oddly shaped) the best thing any pool owner could do FIRST is to install an automatic pool cover. A nice cedar-wood bench can be built over the pool cover roller using a microlam beam. Most pool heat loss is by evaporation. With such automatic cover (totally different from a winter type cover), which is only rolled back with a switch when they want to swim, the size you mentioned should be ok, otherwise it seems too low to me for NY. Automatic pool covers also cut chemical costs to almost nil, pool filter pumps don't have to run much, even small children can walk around the pool unsupervised, no leaves in the pool and the terrible humidity damage problems in indoor pool installations are solved.
I am currently planning to install an AquaCal WS05 for a smaller 12x24 outdoor pool, plan to have its source water in series from my 5-ton open-loop geothermal heat pump cooling my Delaware home, but even then plan on the benefits of a motorized cover.
Has anybody integrated the controls for such 2 GT systems in series? My open loop system gets its source water from a Franklin SubDrive 75 variable -speed well pump which also does dual duty for my lawn sprinkler system, outdoor faucetts, etc. Best investment ever made since propane costs in Delaware are very high. COOP Electric is only 12 ct/kw here.

Last edited by a moderator: Jan 2, 2015